The air around us

Although we cannot see it, there is air all around us. If you move your hand in front of your face,you can feel the air moving as your hand pushes through it. Blow on your hand and you will feel the air coming out of your mouth. We could not live without air. We take it into our bodies blowing it out again through our noses and mouths. Air is everywhere and it gives life to every living thing. When air moves, we call it "wind".

When we say that a bottle is empty and has nothing in it, that is not really true, for the bottle is full of air. Push it under water and you will see the air coming out of it as the water goes in.

Although we cannot see it, air has some strength. For example, it stops a piece of paper from falling as quickly as¹ a piece of wood. Hold a piece of wood in your left hand and a small piece of paper in your right hand and drop them together. The wood will fall quickly, but the paper will fall much more slowly. The two pieces, dropped together, will fall on the ground at different times. This is because it was more difficult for the paper to push through the air than for the piece of wood. But if you put the paper on the piece of wood and drop them again, they will fall together and reach the ground at the same time, because the wood pushes the air out of the way² and the paper falls as quickly as the wood does. This shows us that falling things must push their way³ through the air.

This is very useful to airmen. When an accident happens to an airplane in the air, such as the airplane catching fire,the airman can jump out and can fall slowly to the earth by his parachute. The airman held by a parachute does not fall quickly to the ground, he goes down slowly.

Airplanes can fly because of the strength of the air. Airplanes are bigger and heavier than motor cars. How, then, can they keep up in the air without falling? The answer is this: as the engine pulls the airplane through the air, the air presses on the under part⁴ of the wings and this holds up the airplane. When an airplane starts from the ground, first of all⁵ it runs on its wheels like a motor car, going more and more quickly. When the speed is great enough⁶, the pressure of the air under its wings lifts it off the ground and it rises like a bird.

Air is a gas. Things called 'solids' keep the same shape when you put them in a vessel. A piece of wood or a stone do not take the shape of the vessel. Things called 'liquids' do not keep their shape in a bottle or a cup. They take the shape of the bottle or the cup, but they keep their size. Put some water into a bottle or a cup and it will take the shape of the bottle or the cup. Air is not a solid, for it changes its shape. And it is not a liquid because it changes its size as well. That is why we say it is a gas.

Date: 2015-12-13; view: 429; Нарушение авторских прав